1. Field of the Invention
The present invention relates to a method of producing a semiconductor substrate having an SOI structure (silicon On Insulator) formed by contacting two wafers integrally, which have different thermal expansion coefficients, and more particularly to a method of producing a semiconductor substrate having an SOI structure formed by heat bonding a single crystalline silicon wafer (hereafter referred as a silicon wafer) and a quartz wafer integrally.
2. Description of the Prior Art
Semiconductor substrates having an SOI structure formed by heat bonding two wafers integrally are quite advantageous as start-ins material for manufacturing integrated circuit devices. For example, in recent years these semiconductor substrates have been used widely as structural substrates for forming liquid crystal devices for HD televisions such as TFT transistors and their peripheral drives and control circuits.
Japanese Patent Application Publication (Koukoku Kohou) No. 13155/1975 discloses, for example, as a means of producing such semiconductor substrates having an SOI structure, a technique for heat bonding two silicon wafers firmly together by heating these two wafers in close contact therebetween at a high temperature (for example about 1,200.degree. C.) under an oxidizing atmosphere.
When a silicon and a quartz wafer are brought in contact with each other, according to experiments conducted by the present inventors, it is observed that the wafers are joing together relatively strong even at room temperature without heat bonding, if the contact surfaces are polished and cleaned thoroughly. However, the adhesive strength between the bonded wafers is still insufficient if the outer surface of the silicon wafer is subsequently subjected to mechanical surface grinding to thin the silicon wafer.
It is further observed that heat treating the wafers results in stronger bonding, however, the heat treatment may cause thermal strain to occur in one of the wafers after the wafers are closely contacted together, or during cooling treatment after full heat bonding together of the wafers. Such temperature variations may cause cracking, or partial separation between the wafers, or the like, causing possible damage to the wafers due to differences in thermal expansion coefficients (i.e. the thermal expansion coefficient of silicon=2.33.times.10.sup.-6 verses the thermal expansion coefficient of quartz glass=0.6.times.10.sup.-6).
To obviate such disadvantages, there are existing suggested methods to form an amorphous silicon (a-Si) film directly on a glass substrate corresponding to a quartz wafer by vapor deposition, or other means instead of contacting two wafers as mentioned above. Another existing method includes forming a polycrystal line silicon film on a glass substrate, for example, by a CVD method. However, in such an amorphous silicon film or polycrystalline silicon film, the mobility of electrons in the substrate is greatly restricted and therefore the substrate is not satisfactorily suitable for HD televisions and extra-high frequency and extra-high luminance display devices to be developed in the future.
in addition to the restriction of the mobility of electrons in the substrate, there is a further technical limitation in the density of pins to be connected to external drive circuits making packaging quite difficult, particularly if TFT transistors are embedded in high density when forming TFT-LCD (thin-film transistor liquid crystal display), or the like, by applying multi-layer formation techniques such as a-Si film formation mentioned above.
On the other hand, when a single crystalline silicon wafer is used instead of an a-Si film formed on the glass substrate, the mobility of electrons can be increased in comparison with the case of the a-Si film. Thus, according to this method there will be no packaging problem, since the driving circuits can be integrally formed around the TFT region by heat bonding the polycrystalline silicon wafer onto the quartz glass plate (wafer). However, in the structural substrate for forming a TFT-LCD with single crystalline silicon wafers, the above-mentioned technical problem concerning differences in thermal expansion coefficients still remains unsolved.